Modern bridges include separate elements coordinated together to create a strong and durable structure designed to last for many decades or longer. One of those bridge elements is a bridge deck, which provides the support for and the surface upon which man and machine may traverse over whatever the bridge is spanning. In a vehicular bridge, the primary function of the bridge deck is to support the vehicular traffic safely on a smooth and reliable surface, but also to receive the vehicular vertical loads and distribute those loads to the steel superstructure of the bridge.
A bridge deck is typically continuous along the span of the bridge and continuous across the width of the span. In most applications, the bridge deck is made of composite materials, with the steel superstructure supporting it through positive attachment to the girders, such as using shear connecters to attach the concrete deck slabs to steel girders. In such cases, the deck serves as part of the top flange in the composite section and is utilized to provide strength and stiffness to the bridge.
A bridge deck is subjected to various 3-dimensional forces, including local flexural bending of the slab spanning over the girders in the transverse direction caused by the vehicle wheel loads, and is subjected to longitudinal stresses caused by flexure along the bridge span. The deck, when positively attached to the girders, provides continuous bracing of the top flange in the finished structure, and provides stability to the overall bridge system. The deck also acts as a horizontal diaphragm that is capable of transferring lateral loads, such as wind or seismic loads, to the supports.
Often, especially in the United States, reinforced concrete deck slabs are used as the deck for steel bridges. Concrete deck slabs can be constructed with cast-in-place or precast methods, and typically include mild steel reinforcement in the longitudinal and transverse directions. Although not common to typical steel bridges, concrete decks can utilize post-tensioning steel in addition to the mild steel reinforcement in an effort to provide additional strength and durability. While well understood and common in bridge building, the use of concrete slabs as a deck presents tremendous weight loads on the primary structures of a bridge, such as the primary cables in a suspension bridge or counter-lever steel beams in a counter levered bridge. The designers of bridges using concrete decks take theses loads into account when they design the construction of the bridge.
An alternative to a concrete deck is an orthogonal-anisotropic deck, or as more commonly known an “orthotropic” deck, which is typically made of steel. Orthotropic Steel Decks are referred to in the construction industry as an “OSD” systems and are used in many of the world's modern bridge structures. Use of OSD system does a good job to distribute vehicular traffic loads across the extended deck surface, and provide stiffening of the relatively slender plate elements of an OSD that are under continual compressive and active loading. One well-known example of an OSD based bridge is the recently replaced San Francisco Oakland Bay Bridge which replaced a common concrete slab deck arrangement after an earthquake destroyed a portion of the bridge in the 1980s.
An OSD system consists of a flat, thin steel plate, stiffened by a series of closely spaced longitudinal ribs with support by orthogonal transverse floor beams. The OSD is efficient in that it is integral with the supporting bridge superstructure framing as a top flange common to both the transverse floor beams and longitudinal girders. This results in increased rigidity and material savings in the design of these components. As with other conventional steel-framed construction, loads are generally transferred by the floor beams transversely to the main load carrying system, such as longitudinal girders. This design is far more cost effective than the common use of concrete slabs with steel rebar reinforcing. Instead, orthotropic decks are fairly hollow on the inside and make bridge decks lighter which reduces the weight requirements of the bridge super structure. However, in additional to reduced weight requirements of the super structure, a defining characteristic of the OSD bridge is that it results in a nearly all steel superstructure which has the potential (with minimal maintenance) to provide extended service life and standardized modular design, as compared to more conventional bridge construction.
As is widely recognized, OSD construction has tremendous potential for use in short to medium span “workhorse” girder bridges when located on a high-volume roadway where accelerated construction or extended service life is required. Further, there is a recent trend in the foreign countries, and especially in Asia, towards using bridge systems that are more rapidly constructed to provide traffic solutions that offer long-term durability and economy with the goal of 100 years of service life. Part of the popularity of the OSD bridge is that it can be constructed quickly because most of the components may be prefabricated in high volume. Additionally, complete future re-decking is rendered unnecessary, which minimizes major traffic impacts in the future. In highly populous regions, such as China, the minimization of traffic impact is paramount once a bridge system is put into service. Furthermore, the OSD provides a smooth continuous riding surface durable against deicing salts with minimal joints to prevent leakage and protect the other bridge components.
However, the bridge construction industry recognizes that OSD bridges have not been problem-free historically, and they present unique challenges in terms of design and construction as compared to conventional bridge construction. Fatigue cracking has been observed more frequently in OSD systems resulting from the complicated weld demands combined with stresses that can be more difficult to quantify and, in particular, which were found in early designs which attempted to overly minimize plate thicknesses to reduce weight. In addition, design loading is determined by live loading (moving vehicles) versus dead loading of the span which requires a precise loading design strategy, and such cyclic live loading dominates the design because fatigue will be the controlling limit for a particular bridge design. Hence, fatigue avoidance in OSD systems requires careful consideration as these systems.
Early analytical tools were limited in their ability to quantify the stress states in these details and the early experimental fatigue resistance database was limited. Moreover, the fatigue performance of many of these details can be sensitive to fabrication techniques. Design and detailing practices relied heavily on experience gained through trial and error. Unfortunately, many trials were unsuccessful, and reports of cracking have occurred in re-decking projects where the interactions between new OSD and existing concrete structure were difficult to account for, and created questions among users especially in the United States as to the long-term effectiveness of OSD systems in the highway infrastructure.
The potential for cracking at the rib-to-deck plate weld is indicative of this problem. Whereas this one-sided weld was once a source of performance issues, it is now executed with a vast increase in consistency and performance by using a partial joint penetration paradigm controlled penetration percentages, and with no tolerance for melt-thru in the welds. Cracking is also possible at the rib-to-floor beam intersections, where 3-dimensional stresses are generated by the in-plane flexure of the floor beam response combined with the out-of-plane twisting from the rib rotations. All of these details have been the subject of extensive research efforts over recent decades, providing better understanding of performance and proper design of OSD systems.
In response to these stress issues, the construction and fabrication techniques employed are very important to the successful use of orthotropic steel bridge decks. Orthotropic steel decks typically require detailed construction specifications and special quality control procedures during fabrication. Current designs typically are not standardized, and thus repetition does not currently help to improve construction and fabrication techniques, however many welding strategies with respect to rib to deck connection and other OSD elements have been refined over the years to ensure the proper distribution of stress across and to and from the decking.
During constructions of an OSD bridge deck, deck plating meeting various ASTM codes are cut to size in accordance with size and design of the bridge and are joined together using either an open or a closed set of steel ribs. The open type of rib arrangement consists of ribs usually made from flat bars, bulb shapes, inverted tee-sections, or angled plate sections. In the closed rib arrangement, the ribs are typically formed into trapezoidal, U-shaped, or V-shaped sections.
The closed-rib system is the preferred system relative to open-ribs for a number of reasons. First, it has much higher flexural and torsional rigidity. The high torsional rigidity contributes to better distribution of concentrated transverse loads and, consequently, to a reduction in stresses in the deck plating. Fewer welds, less distortion, and reduced steel weight are further advantages. However, a complication of the closed rib system is in the execution of the one side partial penetration weld for the rib connection to the deck plate. Various stress and fatigue testing of OSD systems over the years has necessitated the use of a partial penetration weld on the outside of the closed-rib where it attaches to the deck plate (see FIG. 1). This fatigue sensitive weld requires care for fabricators to execute with consistent quality. Also, due to its geometry and inherent torsional strength, closed rib decks are subject to local secondary deformations and stresses that make them vulnerable to fatigue at the intersection at the rib to deck. Furthermore, field splices of the ribs are also more complicated, and this system requires tolerance control in fabrication and erection to ensure proper fit at the splices.
In either case, open or closed, ribs are arranged parallel to the vehicle traffic direction and positioned orthogonally with respect to transverse floor beams, and due to manufacturing costs, trapezoidal shaped rib sections are the most common type of rib shape specified in closed OSD systems because they are more easily pre-fabricated in repeatable sections and they may be lifted into place as a section when completed.
As indicated above, ribs are welded to deck plating using a partial penetration technique. Generally, partial penetration welds are avoided in bridge design and construction because, depending on the joint configuration, associated stiffness, and the applied stress, such welds can be a fatigue concern. In fact, use of the partial penetration weld in the rib to decking is an exception to general AWS provisions to weld several types of joints that will be subjected to tension in the root of the weld. This is why the penetration, melt-through, and root gap must be carefully controlled during weld production in the rib to deck joining. Further, over years of observation and laboratory testing, welds joining rib legs to the underside of the decking plating are the most common area prone to fatigue cracking due to plate deformation, which is caused by the active loading of vehicles moving over the deck surface. Hence, strict quality controls over the partial penetration welds in bridge OSD systems is paramount to bridge construction success.
In melt-though, a small amount of weld material oozes into the backside of the weld joint during the welding process. With blow-through, the weld material spatters through the weld joint. Both of these conditions create sites of potential crack initiation and scrutinized during weld examination, especially blow-through which can be avoided with proper welding technique. It is known that a moderate amount of melt-through is permissible. See FIG. 3. The issued in particular is that the “weld throat” or distance from the “weld face” to the “weld root” must be long enough to ensure a sufficiently strong bond between the deck plate and the rib leg. In addition, testing and experience has shown that a penetration amount of less than 70% provides insufficient weld strength, but a weld penetration amount of greater than 80%, and especially 100%, may lead to fatigue cracking initiated from the weld root when exposed to out-of-plane bending moments. Hence, because bridge plate decking is exposed to continuous cyclic loading from vehicle traffic, it is critical that all partial penetration welds connecting the plate decking to the closed support ribs achieve a minimum of 70% penetration, but not over 80% penetration.
Rib to deck welding should be monitored during any bridge construction project, and ultrasonic penetration testing should be conducted throughout the fabrication process for each portion of the decking constructed to ensure weld penetration compliance. However, ultrasonic testing is time consuming and conducting more testing than is necessary causes unnecessary delays and cost. Additionally, while ultrasonic testing is useful for detecting weld defects and various systems are available for such testing, detecting the penetration of a weld using current ultrasonic testing systems is difficult and not optimized to detect the penetration percentage of welds in a rib to deck weld scenario. In particular, conventional ultrasonic systems (i.e. non-phased array systems) do not have the beam control and resolution to accurately measure the amount of penetration in a weld. First, probes in conventional ultrasonic systems only offer fixed angles of beam profile, and the beam cannot be focused in a real-time analysis. So, penetration height cannot be accurately determined in many instances. Second, conventional ultrasonic systems do not allow a user to focus the beam to provide the necessary resolution to discern certain weld anatomy elements that are required to calculate the penetration of the weld.
In addition, even with phased array ultrasonic systems the time required to do a manual examination of a weld seam along a rib would be impractical. For example, to manually examine a 10-inch weld seam to determine the level of penetration a coarse analysis could be done at 1-inch increments. Data slices or sections would be sized, including the angle to determine height, by moving a beam focusing cursor through the weld at that single scan position, or a 6 db drop (to remove non-substantive noise and defects), and other techniques to determine the penetration. However, each such manual examination would, if performed by a skilled operator would take 20-30 seconds for each slice. So, 10 slices in a 10″ weld would take 200-300 seconds or 3 to 5 minutes. However, an accurate determination of a weld seam in order to determine whether the weld seam passes a particular building code specification, such as what is the average penetration depth over a specified distance, requires taking many more samples. A typical sample interval to achieve the data necessary to make a code compliance determination is 0.039 inches separating each sampling slice or section. Expanding on the above estimate, a manual analysis of a 10-inch weld seam using this sampling resolution would require 12.5 hours to 20.8 hours to complete. Extrapolating further, assuming that a road has 4 lanes with 4 ribs per lane (i.e. 8 seams per lane), and the bridge is 1 mile long, the resulting weld seams requiring a single manual inspector would take 9-15 years, working 24 hours a day in a perfect labor and contracting situation. Hence, even for a relatively short bridge of 1 mile, a manual inspection of weld penetrations on such a bridge even if the number of inspectors was increased would be impractical to the point of never being accomplished in any economically viable manner. The result is that only imprecise sampling using manual testing is currently done on OSD systems which leaves bridges with mostly untested rib to deck weld seams, the integrity of which is the most fatigue prone element in any bridge construction project.
Therefore, what is needed is a practical testing system, such as using a phased array testing system, that can quickly and accurately indicate the penetration of a weld in an OSD rib to deck joint.